If the OP puts the answer in the body of the question, or if the answer ends up in the comments on the question, leave a comment asking the OP or the commenter to put the information in an actual answer.

If that doesn't happen within a couple of days, feel free to put the information in an answer yourself. To prevent the appearance of reputation point whoring/stealing, it'd be best to mark the answer as Community Wiki. This way, at least, people can vote on the answer and future readers can find the better solution.

See also:

* http://meta.stackoverflow.com/questions/1555/mark-a-comment-as-answer-to-a-question
* http://meta.stackoverflow.com/questions/54762/flag-a-comment-as-should-be-an-answer
* http://meta.stackoverflow.com/questions/77900/marking-comment-as-an-answer
* http://meta.stackoverflow.com/questions/108969/best-way-to-deal-with-questions-answered-by-the-op-in-the-question-instead-of-in

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Of course, that all assumes the answer is worth keeping around. If the solution is something obvious/trivial like "I rebooted my phone" then the proper course of action is to close the question as "Too Localized".